1. Field of the Invention
The instant invention generally relates to engines having a combustor in which a fuel and oxidizer reactant mixture is continually mixed with product gases to obtain stable combustion, and to methodologies for achieving improved combustion stability and increased combustion efficiency in such engines. More specifically, the invention relates to methodologies for improving interpenetration of reactant mixtures and product gases within combustors and thereby obtaining improved combustor performance.
2. Description of the Related Art
In traditional gas turbine engines, a compressor first compresses air, which then mixes with a fuel like jet fuel, kerosene, natural gas or propane to produce a reactant mixture. The reactant mixture burns in a combustor to produce combustion product gases. The heat that comes from the burning reactant mixture expands the combustion product gases. As these hot product gases flow at high speed through a bladed turbine, a torque is produced on a turbine shaft. The torque is used to drive the compressor and possibly one or more external implements like a generator.
In contrast to a traditional gas turbine engine, a traditional ramjet engine has no moving parts, and in particular has no compressor or turbine. As known in the art, the traditional ramjet engine instead achieves compression of intake air by the ram pressure that develops in a diffuser from the relative difference in velocity between the engine and the intake air. The diffuser leads to a combustor and compressed air from the diffuser mixes and burns with fuel in the combustor to produce product gases in a similar manner as the traditional gas turbine engines.
However, conventional ramjet engines can only be used when the forward speed of the engine is sufficiently high relative to the surrounding air to produce suitable ram pressure in the diffuser which is needed for acceptable performance. To overcome this limitation of conventional ramjet engines, rotary ramjet engines have been proposed. The rotary ramjet engines configure helical-shaped compression-combustion-expansion channels, which provide the ramjet flow path, in a rotor. The rotor rotates about a central axis to achieve the high relative velocity needed for effective ram compression. One known example of a rotary ramjet engine includes the helical-shaped channels formed by an appropriately shaped radially outward surface of an inner supersonic rotating rotor that faces toward a radially inward surface of an outer stationary stator. Another known example of a rotary ramjet engine is a so-called “inside-out” rotary ramjet engine, which includes a stationary stator having a generally annular, radially outward surface centered about a central axis. A rotor is rotatably supported about the central axis and is concentrically axially aligned with the radially outward surface of the stator. The rotor includes a helically extending ramjet flow channel formed therein. The flow channel in the rotor is disposed radially outwardly of the radially outward surface of the stator such that the rotor orbits about the central axis positioned on the radially outward surface during operation of the engine.
Traditional gas turbine engines, conventional ramjet engines, rotary ramjet engines, and other engines that operate on the internal combustion principle have one or more combustors in which reactant mixtures are continually interpenetrated with product gases. The fuel and compressed air entering the combustor are often referred to as reactants. Often the initially separated fuel and compressed air are mixed together to form the reactant mixture prior to combustion in the combustor. During operation of such engines, combustion is commenced in the combustors, and the flow of the reactants into the combustors and the mixing of the reactants with combustion products generally maintains continual combustion in each combustor. There is a general desire to be able to flow as much of the reactant mixture as possible into a combustor of a given volume while maintaining stable combustion and maximizing the combustion efficiency, or to minimize the volume of a combustor while maintaining stable combustion and maximizing combustion efficiency for a given flow rate of reactants.
Additionally, because continuous combustion is sought within the combustors, hot combustion product gases are also present in the combustors along with the reactant mixture. Interpenetration and associated mixing between the hot product gases and the reactant mixture within each combustor is important to maintain continual combustion in the combustor. Although the product gases are no longer combustible, it is important for the product gases to be present in the combustor, and it is important to interpenetrate the product gases and the reactant mixture within the combustor for purposes of maintaining continuous combustion in the combustor. Achieving proper interpenetration and associated mixing between the product gases and the reactant mixture within the combustor is an important consideration for effective combustor design.
Rate of interpenetration achieved between the product gases and the reactant mixture is another factor that is typically important in combustor design, since the rate of heat release due to combustion is in part determined by the rate of interpenetration. For example, if the rate of interpenetration is too slow in comparison with the rate at which the reactant mixture enters the combustor, then the rate of heat release may not be sufficient to maintain continuous, stable combustion within the combustor. This can cause combustion to cease, often referred to as “flameout”.
Another factor that is typically important in combustor design is that the interpenetration should distribute the hot product gases throughout most of the combustor, so that the amount of the reactant mixture flowing through the combustor that is burned before the reactant mixture exits from the combustor is maximized. This is typically important for maximizing combustion efficiency.
As such, there is a strong desire to achieve effective interpenetration and associated mixing of the hot product gases with the incoming reactant mixture to avoid flameout and maximize combustion efficiency. Various efforts have been made in the past to alleviate conditions that result in flameout within the combustors and to thereby stabilize combustion within the combustors. For traditional gas turbine engines and conventional ramjet engines, many such efforts focus on aerodynamic stabilization through geometric design of the diffuser and combustor to control the flow pattern of the reactant mixture and the product gases within the combustor. In effect, aerodynamic stabilization designs the internal geometry of the combustor and various parts within it so that inertia of the reactant mixture entering the combustor provides for interpenetration of the reactant mixture and product gases within the combustor. However, even in common engines such as traditional gas turbines, such aerodynamic stabilization methods can only ensure continuous combustion over a limited range of reactant mixture flow rates and relative fuel-to-air mass ratios than would be desirable. It is therefore desirable to pursue other methods for effectively interpenetrating reactant mixtures and product gases and achieving continuous combustion in addition or as an alternative to methods based on traditional aerodynamic flame stabilization.
Moreover, in some types of engines, such as the rotary ramjet engines as described above, aerodynamic stabilization methods may be generally insufficient for stabilizing combustion to avoid flameout over a useful range of design or operating conditions. In particular, in rotary ramjet engines the reactant mixtures and product gases in the combustors may be subjected to large accelerations as a consequence of the rotating nature of the combustors, and thereby become subjected to large forces that can affect the flow and resulting interpenetration pattern of the reactant mixture and product gases within the combustor. Notably, the density of the hot product gases is substantially lower than that of the reactant mixture entering the combustor. Due to rotation in the rotary ramjet engines, the resulting centrifugal forces that can arise within the reactant mixture and product gases can inhibit adequate interpenetration needed to maintain stable combustion. As a consequence, rotary ramjet engines especially are in need of additional methods beyond aerodynamic stabilization to maintain the continuous combustion therein over a broad range of design and/or operating conditions.